The abdominal skin of the frog, a widely employed model for the mammalian kidney, carries out transepithelial active sodium transport driven by oxidative metabolism. Despite intensive study, it is not known whether the sodium pump functions essentially as a "voltage source", responding directly to the serosal membrane electrical force or as a "current source", responding only to cell sodium activity, or whether its behavior is intermediate. This problem will be approached by dynamic electro-physiological studies while altering the rate of active transport by perturbation of the transepithelial electrical and/or chemical potential difference. In addition to standard voltage clamp techniques for the characterization of trancellular and paracellular conductances, we shall use fine open-tip microelectrodes to measure mucosal and serosal membrane electrical potentials, and Na+ liquid ion-exchanger microelectrodes to measure cell sodium activity. Prompt observations should permit characterization of the dynamic response of amiloride-sensitive transcellular current flow, the membrane conductance ratio, and cell sodium activity. Steady state observations should permit the measurement of trancellular Na+ current, the partial ionic conductances, the electromotive force of Na+ transport, the permeability of the mucosal membrane, and the rate coefficient of the Na+ active transport mechanism Qpump. The results should provide a framework for elucidation of mechanisms of regulation of Na+ transport in health and disease, and in the presence of drugs and hormones.